Today, most computer systems include one or more magnetic recording devices onto which digital data from the computer system is stored. In this manner, computer programs, textual information, graphics, and numerical data can be stored on a permanent basis for subsequent retrieval.
One such magnetic recording device is known as a Winchester, rigid, or hard disk drive. In a hard disk drive, data is typically stored on an inflexible magnetic disk. The disk is circular, and number of concentric circles or rings, commonly referred to as "tracks", are described upon the surfaces of the disk. It is within these tracks that data is stored as a series of binary bits of information.
A transducer, also known as a "head", is used to write the data from the disk. In many instances, the same transducer is also used to read data from the disk. Commonly, a number of disks are stacked together and are rotated about a spindle. There are as many heads as there are disk surfaces. In other words, a separate head is used to access each surface of the disks.
A servomechanism is used to locate the head in reference to fixed radial locations over the disk surface. It is the function of the servomechanism to reposition the head from one radial location to another radial location. When the computer directs the hard disk drive to position the head on a track which is different from the track wherein the head is presently positioned, this is lo known as a "seek".
Typically, data is written to the first available space. This results in data being written onto the tracks in a semi-random process. Moreover, a computer program typically performs numerous read and write operations per task. Thus, multiple seeks are often required for each time the disk is accessed. Since data cannot be written onto the disk nor can data be read from the disk throughout the duration of a seek, it is of great importance that the seek be completed as rapidly as possible and that fewer seeks are performed to accomplish the same amount of work. It is also imperative that the seek be performed accurately in order to assure that the correct data is accessed.
In the prior art, there exists essentially two different mechanisms for effectuating seeks --dedicated and embedded servo systems. In both types of servo systems, the heads are mechanically arranged such that the relative position among the heads is permanently fixed. In a dedicated servo system, one of the heads, known as the "servo" head, is dedicated to reading prerecorded servo information from a disk. The remaining "data"heads are devoted to the recording of customer or utility data. Because of the mechanics, the radial position of the servo head automatically establishes the radial position of the data heads. The positional accuracy of the data heads is dependent on the thermal and structural integrity of the mechanics connecting the data heads to the servo head.
Sequential data is generally organized such that it is written under all the data heads in a "cylinder" before changing cylinders. Since the radial position reference is completely dependent on the servo head, the switch between data heads for a particular cylinder is virtually instantaneous, as the switching is done electronically. Thus, the elapased time from start to finish required to write sequential data throughout an entire cylinder is that for as many revolutions as there are data heads. Upon completion, the servo head repositions to the next sequential cylinder of data tracks, and the disk data transfer continues. The rotational position where data transfer continues on the next cylinder is skewed from that of the last data in the previous cylinder by the amount the disk would spin during the seek. This keeps the data transfer delay incurred while changing cylinders at a minimum. The total delay incurred over transfers spanning multiple cylinders is the seek delay required between cylinders.
However, the dedicated servo system has two inherent problems. Dedicating an entire surface for radial positioning reduces the disk surface area available for a user's data storage. This tends to reduce the capacity of the disk drive. Furthermore, improvements in magnetic storage technology lead to ever thinner track widths. This places tighter constraints on the mechanics to maintain constant relative position among the heads. Any misalignment among the servo and data heads could impair the ability of the disk drive to retrieve previously recorded data. Dedicating a recording surface solely for servo information plus the mechanics for rigid head alignment renders the dedicated servo approach increasingly impractical in today's marketplace.
The embedded servo system eliminates the need for a dedicated servo surface by allocating a small portion of each data track to record servo information thereon. Small pieces of servo information are equidistantly spaced around every track. The electronics and firmware dynamically separate the servo information from the data. This allows spin speed and radial position corrections to be made based on the head where data transfer is occurring. Mechanical misalignment of heads becomes relatively insignificant and the drive storage capacity increases by a large fraction of a recording surface. The requirements for mechanical and thermal integrity are significantly reduced.
Nevertheless, embedded servo systems suffer a significant performance penalty in that the repositioning required for each head switch, delays data transfer by a fraction of a revolution. Whereas the dedicated servo drives only incur delay on the last data head of a cylinder for an incremental seek, the embedded servo drive incurs a delay for each and every head switch in addition to the incremental seek at the end of the cylinder.
Therefore, there is a need in the prior art for minimizing the cumulative amount of data transfer delay due to seeks between cylinders and head switches without sacrificing accuracy or diminished storage capacity. It would highly preferable to achieve this goal without significant increase in production costs.